Great thread! I'm reviving it to add some favorites not mentioned yet:
Carl Vine - Piano Sonata No. 1 (1990), here played in a PHENOMENAL performance by Michael Kieran Harvey, to whom the piece is dedicated:
The Vine Piano Sonata No. 1 is possibly my favorite piano sonata of all time.
Peter Sculthorpe - Irkanda IV (1961)
There is something haunted and dangerous about this piece as if the violin is stranded in the middle of the vast and hostile Australian outback.
Toru Takemitsu - To the Edge of Dream (1983)
Takemitsu's orchestration on this piece is stunning.
Edgard Varese - Arcana (1925-1927)
Fans of Jerry Goldsmith's score to Planet of the Apes will like this one I think.
Guillaume Lekeu was a promising young composer mentored by Cesar Franck who contracted typhoid fever and died the day after his twenty-fourth birthday. I claim no great knowledge of him or his works, and in fact the piece I am going to link you is the only one of his that I've heard. It's his Adagio for String Orchestra, a beautiful and melancholic work written after Franck's death, and an excellent demonstration of just what was lost when he died.